I think it's a personal philosophy if you price per cake/size versus per serving. It seems, at least in my family and circle of friends that the size of a "serving" is not realistic. We EAT cake! So...if I chose to post prices...I'd probably opt for sizes.
Basically pricing for size or serving is the same thing. For example...according to the Wilton chart (which is the chart I go off of) a 6" round cake feeds 12 people. You can either tell someone "the cake feeds 12 and at $3/serving the total is $36" or you can say "all my 6" cakes start at $36". It really doesn't matter if they want the cake to feed 1 person or 12 - the cost is still $36. But this is where educating the customer comes into play. They are going to call you and think "I can eat a 6" cake all by myself which means a 6" cake serves 1 - so it should cost $3". If you base your pricing off sizes I would put a little pice of text next to the size saying "feeds 10 - 12" or "20-24" or whatever the servings would be so they order accordingly.
I mostly do wedding cakes or other larger orders, so people don't call me saying "I need a 10" cake"; they call saying "I need a cake to feed 150 people" and I direct them to the size of cake tiers and number of tiers they will get.
I guess it depends on the type of orders you get and type of events you go after.